September 04, 2014

What about the Other Weight Loss Diet Study??

The same day the low-fat vs low-carb study by Bazzano and colleagues was published, the Journal of the American Medical Association published a meta-analysis that compared the effectiveness of "named diet programs". Many people have interpreted this study as demonstrating that low-carbohydrate and low-fat diets are both effective for weight loss, and that we simply need to pick a diet and stick with it, but that's not really what the study showed. Let's take a closer look.

Johnston and colleagues sifted through PubMed for studies that evaluated "named diet programs", such as Ornish, Atkins, LEARN, Weight Watchers, etc (1). In addition, the methods state that they included any study as low-carbohydrate that recommended less than 40% of calories from carbohydrate, was funded by the Atkins foundation, or was "Atkins-like". These criteria weren't extended to the low-fat diet: only studies of name-brand low-fat diets like the Ornish diet were included, while the meta-analysis excluded low-fat diet studies whose guidelines were based on recommendations from government and academic sources, even though the latter group represents the majority of the evidence we have for low-fat diets. The inclusion criteria were therefore extremely asymmetrical in how they represented low-carb and low-fat diets. This fact explains the unusual findings of the paper.

The abstract immediately activated my skeptic alarm, because it states that at the one-year mark, low-carbohydrate diets and low-fat diets both led to a sustained weight loss of about 16 pounds (7.3 kg). Based on my understanding of the weight loss literature, that number seems far too high for the low-fat diet, and also too high for the low-carbohydrate diet.